1. Field of the Invention
This invention relates to the separation of CO2 from liquid absorbents and the recovery of substantially pure CO2 for sequestration or other processing to prevent its release into the atmosphere.
2. Description of Related Art
A number of processes have been developed that employ a liquid sorbent for the capture of carbon dioxide from combustion flue gases emanating from electrical power generation plants and other industrial sources. After the CO2 has been absorbed/adsorbed, it is transferred to a desorption unit where it is typically heated to a higher temperature to release substantially pure CO2 which can then be recovered for further processing. Typical CO2 sorbents contact the flue gases at an initial temperature of from about 30° C. to 40° C., and thereafter the temperature of the CO2-rich liquid sorbent is typically raised to the range of from 100° C. to 125° C. to desorb the CO2 gas. The liquid sorbent temperature can be increased to the desorption range by contact with a heat exchanger.
The energy required to desorb the CO2 from the CO2-rich liquid sorbent must be supplied in the form of heat from other systems in the power generation facility and the hot CO2-lean absorbent liquid must be again reduced in temperature before it can be returned to contact the flue gases. Studies of commercial power generation facilities have shown that even the most effective and efficient systems for the recovery of CO2 from flue gases and recycling of the liquid sorbent result in a significant drop in the overall efficiency of the power generation facility as compared to operating the same facility without CO2 capture to reduce the CO2 released into the atmosphere.
A problem therefore exists concerning the amount of energy consumed by the present power generation systems and methods for the regeneration of liquid sorbents used to capture and recover CO2 from flue gases.
A process is disclosed in U.S. Pat. No. 8,512,660 (the '660 patent) that employs a cyclone separator having pressure controlling means to selectively recover methane from a liquid sorbent that also contains CO2 separated from a biogas. In the absorptive removal of carbon dioxide from a methane biogas using a scrubbing liquid or sorbent, as much as 10% of the methane present in the biogas can also be absorbed. With careful maintenance of the temperature at which the liberation of CO2 begins, effective separation of only methane from the CO2 and methane-rich scrubbing solution can be achieved, e.g., with the first 1% to 2% of liberated CO2, virtually all of the absorbed methane is released from the liquid sorbent. After this temperature-controlled heating, the CO2-rich and methane-containing liquid sorbent is fed to a centrifugal separator for separating the methane gas from the liquid phase, wherein methane and a small fraction of the dissolved CO2 escape via the gas phase. The gas phase is recovered and passed to the absorber unit where it is mixed with the fresh crude biogas. Thereafter, the liquid phase removed from the centrifugal separator, which is a virtually methane-free CO2-rich liquid sorbent, is passed through a second heat exchanger and there it is heated to the required CO2 desorption temperature and the sorbent is thereby regenerated in the desorption unit and recycled to the absorption unit.
In one embodiment of the '660 patent, the pressure of the gas phase in the cyclone separator is controlled, so that temperature fluctuations in the centrifugal separator and changes in the gas loading of the liquid sorbent that is fed can be compensated for by the pressure controller.
Thus, the '660 patent is directed to the problem of separating the 10% or so of methane from the CO2 in a liquid absorbent that has been used to treat a biogas by reliance on the differences in the kinetics and solubility parameters that lead to a much more rapid desorption of methane. The '660 patent employs the conventional prior art method of heating the CO2-rich liquid sorbent to a high enough temperature in the desorption unit to release the CO2. Here again, the energy consumed in heating and cooling the liquid sorbent must be charged against the process when calculating the overall efficiency of the system.
Thus, a need exists for an improved system and method that can receive a stream of CO2-rich liquid sorbent and process it more efficiently than current systems to strip the CO2 from the liquid sorbent to a predetermined concentration for recycling and also recover a substantially pure stream of CO2 with a lower energy consumption than possible using presently known methods.
Specifically, the problem to be addressed by the present invention is how to reduce the energy consumption and improve the efficiency of the CO2 capture and recovery process in a conventional combustion flue gas by reducing the temperature to which the CO2-rich liquid sorbent must be heated to desorb a predetermined proportion of the absorbed CO2 so that the CO2-lean liquid sorbent can be recycled to contact the flue gas.